Biodegradable polymers have been used, for example, as excipients in sustained release compositions for drugs. See, e.g., U.S. Pat. Nos. 3,773,919 and 4,767,628. Examples of such polymers are copolymers of lactic acid and glycolic acid, which are made by either the polycondensation of lactic acid and glycolic acid, or by ring opening polymerization with lactide and glycolide. See, e.g., Drug Carriers in Biology and Medicine, ed. Gregoriadis, G., pages 241-245 (Academic Press, London 1979).
International Publication No. WO 94/15587 describes sustained release ionic conjugates of polymers and drugs. As the basic drug is ionically conjugated to the acidic polymer, increasing the acidity of the polymer is important to facilitate the formation of the conjugate.